In automotive vehicles of the passenger delivery van type, it has become common practice to provide the vehicle body with a relatively large side opening that is located immediately behind the passenger side front door and is opened and closed with a sliding door. The sliding door is typically mounted with hinges on horizontal tracks on the vehicle body for guided sliding movement between a closed position flush with the vehicle body closing the side opening and an open position located outward of and alongside the vehicle body rearward of the side opening. The sliding door may be operated manually as is most generally the case or with a power operated system to which the present invention is directed. In powering the sliding door, a power drive mechanism including a reversible electric motor may be mounted in the vehicle body and connected to operate the door through a cable system as disclosed in U.S. Pat. No. 5,046,283. It is also known to mount a power drive mechanism on the sliding door and effect its sliding movement with a rack and pinion gear as disclosed in U.S. Pat. No. 4,612,729. Such prior power sliding door systems, even where the power drive mechanism is mounted on the side door, require considerable accommodating space and modifications in the vehicle body structure and are not readily installed in an upgrading manner to convert an existing manually operated sliding door to a power operated one. Moreover, both the cable system and the rack and pinion gear drive are positive type mechanical drives between the motor and the sliding door and either the mechanical drive must be interrupted in some manner such as by disengaging a clutch incorporated therein or the motor must be stopped in some manner such as with special control circuitry to prevent the sliding door from closing on a physical object such as a package or a part of the anatomy of a person remaining in the side door opening.